Biodiesel Production Using K–Sr/CaO and CaO Catalysts Derived from Eggshells by Canola Oil Transesterification

Eggshell calcination at 900 °C was used to produce CaO, which was afterward impregnated with K and Sr using KNO₃ and SrCl₂·6H₂O precursors, diluted in methanol, to improve basicity, stability, and catalytic activity. The CaO doping with K-Sr affected the final catalyst's textural properties, alkalinity, and basic strength due to the K⁺ and Sr²⁺ size and incorporation into the CaO lattice. SEM images with elemental mapping showed a uniform K⁺ and Sr²⁺ distribution for the K-Sr/CaO catalyst. However, carbonization modified the basic strength and the number of catalytic sites. The fresh K-Sr/CaO and CaO catalysts presented 92.5% and 46% biodiesel yields, respectively. In the third reaction cycle, the biodiesel yield dropped to approximately 72% and 21%, respectively. In this sense, the method of doping CaO with K and Sr increased the basic strength and number of basic sites for the K-Sr/CaO catalyst, providing higher resistance to leaching compared to the CaO catalyst. Finally, the enhanced conditions were 7.0 wt % catalyst loading, a 12.5:1 methanol/oil molar ratio, 70 °C, and a 1 h reaction time.